Problem: Omar starts counting at 48, and he counts by threes. If 48 is the 1st number that Omar counts. what is the 15th number that he counts?
What is the first number that he counts? $48$ What is the second number that he counts? $\begin{align*}&48 + 3 \\ &= 51\end{align*}$ What is the third number that he counts? $\begin{align*}&48 + 3 + 3 \\ &= 48 + (2 \times 3) \\ &= 54\end{align*}$ What is the 15th number that he counts? $\begin{align*}&48 + (14\times3) \\ &= 48 + 42 \\ &= 90\end{align*}$